Title: Flow-vegetation-sediment interaction in a cohesive compound channel

3 The purpose of this study was to quantify how vegetation influences the flow and sediment processes 4 relevant for the design and management of environmental compound channels. Therefore, we conducted a 5 two-year field investigation in a cohesive two-stage channel, focusing on the flow resistance and net 6 deposition in five sub-reaches with different floodplain vegetation conditions. In the grassy sub-reaches, the 7 cross-sectional blockage factor was the key vegetation property governing the flow resistance, with a 8 process-based model providing reliable estimates under widely variable hydraulic and vegetative conditions. 9 The net deposition of cohesive sediment was best explained by the vegetation height while high stand length 10 and density created supply-limited conditions on the inner floodplain. Our results showed that the two-stage 11 approach offers potential for controlling the sediment processes through appropriate vegetation maintenance. 12 The novelty of this research was that straightforward analyses accompanied with a physically-based 13 parameterization of the floodplain plant stands were successfully used to characterize the flow–vegetation– 14 sediment interaction in a practical engineering application. The results are expected to be helpful in 15 designing and managing comparable channels. 16 Subject headings: Flow resistance; Cohesive soils; Suspended sediment; Vegetation; Drainage; Hydraulic 17 models 18 Introduction 19 There is a growing interest towards environmentally sound hydraulic engineering, which attempts to 20 combine technical requirements and ecological aspects in the management of rivers, brooks, and drainage 21 channels (e.g., Hey 2009). One of the environmentally preferable alternatives is a two-stage channel, which 22 refers to a compound cross-section with an excavated floodplain on one or both sides of the main channel 23 (e.g., Powell et al. 2007; USDA 2007). The floodplain is typically designed to be annually inundated for 24 several weeks to months. The two-stage approach has been adopted e.g. in agricultural drainage (Powell et 25 al. 2007; Västilä and Järvelä 2011) and flood management (Geerling et al. 2008; Sellin et al. 1990) in an 26 attempt to provide both ecological benefits and improved conveyance of high flows. Another focus is often 27 on the management of in-channel transport processes of fine suspended sediment (SS). High loads of SS can 28 lead to excessive sedimentation, which decreases the conveyance and alters the morphology and habitats 29